Power splitter for plasma device

ABSTRACT

The invention concerns a system including a microwave generator and a rectangular guide connected with the generator. The system is adapted to operate in fundamental (H 10 ) or transverse electrical (TE 10 ) mode, and associated with means providing a standing wave pattern. The system also includes many power connectors arranged in the guide at zones of maximum amplitude for one of the components of the electromagnetic field for splitting the generator power. The power connectors are adjusted so that the sum of their reduced admittance levels brought to the splitter input formed by the guide is in a single unit and many sources, respectively connected to a connector of the guide, via insulating means ensuring a power transmission of the connector to the source without reflecting towards the connector and a device adapting impedance of each source, located downstream of the insulating means, between the latter and associated source.

BACKGROUND

1. Field

The present invention relates to the field of microwave devices.

More specifically, the present invention relates to the field of devicescomprising several individual microwave sources supplied from a commongenerator.

2. Description of Related Art

The present invention may especially find applications in the productionof a plasma from a given number of individual plasma sources suppliedwith microwave power from a single power generator.

These individual sources may either be independent in the same chamber(the objective being, for example, to overcome the physical ortechnological limits on the maximum microwave power that it is possibleto apply to a single plasma source) or be distributed in the samechamber so as to allow the extension of scale needed for an intendedapplication. In general, the fields of application of multiple plasmasources may cover not only all the fields already covered by the use ofsingle plasma sources, but also novel fields that cannot be envisionedwith unitary sources (for example for reasons of uniformity, rates,etc.).

The invention relates to all microwave plasmas and discharges, whateverthe pressure range, the microwave frequency, the nature or theconfiguration of the microwave applicator and the presence or absence ofa magnetic field.

However, the invention is not limited to the field of plasmas. It may,for example, also be applied for bonding, drying or curing operationsusing multiple stations and more generally for any operation in whichthe impedance of the system may vary over time from one station toanother.

The microwave field has already been the subject of extensive research.

Several proposals have already been made to supply several individualsources from a common generator.

To divide the microwave power delivered by a single generator, it ispossible to use cascades of 3 dB couplers (division by 2) which areformed, for example, from rectangular waveguides. This solution,although often requiring a very large amount of space, makes it possibleto produce power divisions by N=2^(k), where k represents the number ofsuccessive levels of the cascade. Thus, the microwave power may bedivided by 2, 4, 8, 16, 32, etc. A matched coax/waveguide transition atthe end of each waveguide furthermore allows the microwave power to betransported by means of coaxial cables fitted with standard connectors.

Another widely used solution is to take off microwave power either intoa cavity, or into a waveguide, or into a ring resonator, in whichcavity, waveguide or ring resonator standing waves are created, byantennas placed at the electric field antinodes (regions of maximumelectric field. This solution assumes in general that each individualplasma source behaves as a matched impedance, in other words that itabsorbs all of the microwave power taken off. With such a device, it isthen possible to deliver a predetermined microwave power to eachindividual source.

However, the devices proposed hitherto are not completely satisfactory.

One of the difficulties of dividing microwave power for the purpose ofsupplying plasma sources is that, as a general rule, a plasma sourcedoes not behave as a matched load. This is because the impedance takenback to the input of a plasma source, resulting from the combination ofthe input impedance of the applicator and of the impedance of the plasmataken back to this input, does not generally correspond to a matchedload, that is to say a purely resistive load equal to the characteristicimpedance of the microwave supply line. On the contrary, one may befaced, depending of the type of discharge, the discharge conditions andthe absorbed power, with complex impedance values at the input of theplasma source, values which vary from zero to infinity.

In the case of several plasma sources fed by the same microwavegenerator, there is also the problem of the influence of the impedanceof a source on all of the other sources in the absence of sufficientdecoupling (typically>20 dB) between the supply lines for the variousmicrowave sources.

Thus, immediately after ignition, the input impedance of the source isgenerally much higher than that corresponding when the discharge is inthe steady state. Apart from this variation in impedance for a givensource at the moment of ignition, the power distribution is alsoaffected by the various plasma sources not being ignited simultaneously.Consequently, when turning on a number of plasma sources one isnecessarily confronted with significant imbalances in the powertransmitted to the plasma sources and with the introduction ofconsiderable reflected power into the circuit.

These imbalances, which cause very high reflected power levels, mayprevent the plasma from being turned on in the sources requiring aminimum plasma density, and hence a minimum transmitted power, such asfor example in surface wave plasmas.

Another difficulty involving impedance imbalance arises in the case ofplasmas whose plasma density is, on the contrary, limited to an uppervalue, for example the critical density, as in plasmas using distributedelectron cyclotron resonance. In this case, the entire incident powergreater than the value ensuring the critical density is reflected at theinput of the source and sent back into the microwave distributioncircuit.

Furthermore, impedance imbalances may also be encountered duringoperation, for example should one of the sources fail, or after anintentional or unintentional variation in the operating conditions(composition of the gas, flow rate, pressure, density of the plasma,radio frequency bias, etc.) during multisequence processes.

Finally, in the case of several plasma sources operating in the samechamber, the interference between applicators also results in reflectedpower levels which may disturb the desired power distribution.

Thus, the conventional solutions for microwave power division are eitherexcessively bulky (cascades of 3 dB couplers) or allow only division byprescribed numbers n=2^(k), or require a matched impedance, which is notthe case with a plasma source.

SUMMARY

The objective of the present invention is to improve the microwavesystems comprising several individual sources fed from a commongenerator, so as to eliminate the drawbacks of the prior art.

This objective is achieved within the context of the present inventionby means of a system comprising:

a microwave generator;

a rectangular waveguide coupled to the generator, matched in order tooperate in the fundamental mode (H₁₀) or in the transverse electric mode(TE₁₀), and combined with means ensuring standing wave conditions;

a plurality of power output ports placed in the waveguide in the regionsof maximum amplitude of one of the components of the electromagneticfield in order to provide power division for the generator, the poweroutput ports being adjusted in such a way that the sum of their reducedadmittances brought back to the input of the divider formed by therectangular waveguide is unitary and

a plurality of sources which are coupled respectively to an output portby the agency

of an isolator means ensuring power transmission from the output port tothe source, without being reflected back to the output port, and

of a device for matching the impedance of each source, said device beinglocated downstream of the isolator means, between the latter and theassociated source.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Further features, objectives and advantages of the present inventionwill become apparent on reading the detailed description which followsand in conjunction with the appended drawings given by way ofnonlimiting examples, in which,

FIG. 1 shows the change in the reduced conductance of an antenna,brought back to the input of the waveguide, as a function of the lengthof this antenna in the waveguide;

FIGS. 2 and 3 show cross-sectional views of a waveguide and illustratetwo alternative forms of output ports or antennas on the latter;

FIG. 4 shows an overall schematic view of a device according to thepresent invention;

FIG. 5 shows a perspective view of a guide corresponding to a preferredembodiment of the present invention.

DETAILED DESCRIPTION

As indicated above, the present invention makes use of the combinationof three elements, the first, 100, of which ensures the required powerdivision (preferably, but not necessarily, equidistribution according tothe requirements), the second, 200, ensures independent powertransmission, with no reflection, at each source 400, whatever the inputimpedance presented by each of these sources 400, and a third, 300, animpedance matching device on each source 400 ensures that the power thusavailable is more or less completely absorbed (for example in theplasma) according to the requirements.

More precisely within the context of the invention, the power divider100 is obtained from a rectangular waveguide 110 from which power istapped off, generally in the long side 112 of the waveguide, at points114 separated by one half of the wavelength in the waveguide, i.e.λ_(g)/2.

This wavelength λ_(g) satisfies the equation:

1/λ_(g) ²=1/λ₀ ²−1/(2a)²  (1)

where a is the width of the long side 112 of the rectangular waveguide110 and λ₀ is the wavelength in vacuo of the microwaves.

To provide the required power division, it is possible, for example, tocreate a standing wave of constant amplitude by means of a reflectingplane 130 (a fixed or movable short-circuit) located as that end of thewaveguide 110 on the opposite side from the microwave power feed comingfrom the generator 10. Meeting this constant-amplitude standing wavecondition means that the waveguide 110 allows the propagation of only asingle mode.

Thus, within the context of the present invention, the waveguide 110 isdesigned to operate in the fundamental mode H₁₀ or the transverseelectric mode TE₁₀.

In the case of a power take-off by means of an electrical antenna, it isadvisable for the antennas 116 to be preferably placed at the maximum ofthe electric field intensity.

In the case of a take-off by means of a magnetic antenna 116 (a loop),it is necessary on the contrary, to place these antennas 116 at theelectric field intensity minimum (magnetic field maximum).

To ensure that the total impedance presented at the input of the powerdivision device 100 is equal to the characteristic impedance of thewaveguide 110 (unitary reduced impedance), it is necessary for the sumof the reduced admittances (in the case of electric coupling) of all ofthe antennas 116, brought back to the input of the divider 100, to beunitary. To achieve this result, it is necessary either to adjust thedepth of penetration of the electrical antenna 116 into the waveguide110 or to displace the position of the antennas 116 transversely withrespect to the axis of the waveguide 110, or else to combine these twooptions. The magnetic coupling case may be treated in an equivalentmanner.

As an example, in the case of the microwave power split equally N waysusing electrical coupling, the reduced conductance (real part of theimpedance) (with respect to the characteristic impedance) of an antenna116, taken back to the input of the waveguide, is given by theexpression:

g=1/N  (2)

For a given shape and a given diameter, the length of the antenna 116must therefore be adjusted so as to obtain the impedance correspondingto the desired N-way power division.

An example of the change in conductance as a function of the length ofthe antenna 116 is shown in FIG. 1 (for an antenna 3 mm in diameterwith, at the end, a head 5 mm in diameter and 2 mm in thickness) forantennas placed along the axis of one of the long sides 112 of thewaveguide 110.

In order to reduce the length of the device 100 which, in theconfiguration described above is equal to Nλ_(g)/2, an alternative formof the invention consists in placing two antennas 116 on either side ofthe axis of the long side 112 of the waveguide 110 every λ_(g)/2, as inthe first configuration presented. If g₀ is the conductance of anantenna 116 on the axis, its value g at a distance d from the axis ofthe long side 112 of the waveguide 110 is given by:

g=g ₀ cos²(πd/a)  (3)

In order to reduce the length of the device 100 further, anothercomplementary alternative form of the invention consists in placingpairs of antennas 116, as in the previous configuration, opposite oneanother, each pair on the two faces of the long sides 112 of thewaveguide 110, as shown schematically in FIG. 3. However, this option islimited, in terms of the achievable conductance, by the fact that thefacing antennas 116 must neither touch each other, nor be too close toeach other: the interaction between facing antennas 116 results in anincrease in the conductance of each antenna 116.

Apart from the configuration presented above, based on a rectangularwaveguide 110 with a reflecting plane 130, it is possible to insert justpart of the rectangular waveguide 110 with its antennas 116 into a ringresonator. In this case, it is advisable to arrange for the resonator tooperate in standing-wave mode (and not in traveling-wave mode) and toensure by means of phase shifters that the position of the electricfield maxima of the microwaves in the ring corresponds to the positionof the electrical coupling antennas 116.

Another alternative form of the invention consists in taking the poweroff the waveguide via slots, especially within the context ofapplication to the transmission of power to the plasma sources 400 viawaveguides.

The second element 200 of the invention is intended to ensureindependent power transmission, without any reflection, to each source400. This is achieved by inserting a unidirectional isolator 200 betweenthe output of the antenna 116 of the divider waveguide 110 and theapplicator. Said isolator generally consists of a three-branchcirculator 210 based on ferrites and terminated, on its third branch, bya matched load 212 intended to absorb all the reflected power comingfrom the plasma source 400. To operate this device properly requires aninterbranch isolation generally greater than 20 dB.

The third element 300 of the invention is intended to allow impedancematching to each source 400, so as to ensure that the power thusavailable is more or less completely absorbed in the plasma according tothe requirements. This may be achieved by making use of conventionalimpedance matching devices, such as a trombone line, or a system havingthree plungers. An essential characteristic required of these variouspossible devices is to be able to act both on the imaginary part and thereal part of the impedance. This allows the impedance of the source 400to be adjusted according to the desired plasma conditions (density,length, etc.)

A complete typical power division device according to the invention isshown schematically in FIG. 4. After the microwave generator 10 (andoptionally its protective circulator), this comprises, in succession,the power divider 100 with its movable short-circuit 130 and thetransmission lines to each plasma source 400. Each transmission linecomprises a circulator 210 and its matched load 212 (which absorbs thereflected power), together with the impedance matching device 300 justupstream of the plasma source 400.

The main advantage of the device according to the invention is that itallows a large number of plasma sources 400 to be supplied from a singlegenerator 10. Moreover, this device is produced from simple elements,several of which are commercially available at the present time.

A device of the invention may be used with any type of microwaveapplicator.

An essential advantage of the invention presented is the possibility ofdistributing the microwave power over any number N of antennas 116, iteven being possible for N to be an odd number. The invention also allowsone or more plasma lines to be removed without impairing the operationof the others.

The invention, which prevents any interference between the supplies forthe various plasma sources 400, makes it possible to achieve rapidimpedance matching to each of the plasma sources 400.

Finally, the invention allows particularly compact devices to beproduced.

One particular, but nonlimiting, application example illustrating theinvention comprises a device for dividing the power by 24 (shownschematically in FIG. 5), using the WR 340 rectangular waveguidestandard in which the long side 112 of the waveguide 110 has a widtha=86 mm (width of the short side of the waveguide b=43 mm). At afrequency of 2.45 GHz, the wavelength in vacuo is λ₀=122.45 mm and thewavelength in the waveguide (Eq. 1) is:

λ_(g)=174.4 mm  (4)

As a consequence, the antennas 116 or groups of antennas are positionedalong the waveguide 110 every λ_(g)/2, i.e. every 87.2 mm.

The reduced conductance g of the antenna 116 (Eq. 2) for division byN=24 requires:

G=0.0417  (5)

The corresponding conductance g₀ of an antenna 116 of the same length lplaced on the axis of a long side of the waveguide (d=0), given byEquation (3), for a distance from the axis of the long side of thewaveguide d=26 mm, is:

g ₀=0.123  (6)

The reduced impedance go of an antenna 116 of length l, determinedexperimentally, is given from FIG. 1. The antenna length l correspondingto the impedance value given by Equation (6) is approximately (FIG. 1):

l 12.75 mm  (7)

As a consequence, the power divider 100 divided by 24 thus produced, asshown schematically in FIG. 5 is relatively compact since its totallength corresponds to 5 half-wavelengths (plus the space required forthe antenna output ports and their coaxial connectors).

Of course, each of the 24 transmission lines coming from the divider 100comprises, in succession, an isolator 200 with its matched load 212 andthe impedance matching 300 just upstream of the plasma source 400.

Outside plasmas, the device according to the invention can be applied inany process where impedance variations may arise on one or other of theN applicators supplied independently by the microwave power divider.

Of course, the present invention is not limited to the particularembodiments which have just been described, rather it extends to anyvariant in accordance with its spirit.

Thus, as a variant, it is possible to provide an arrangement of antennaswhich is not symmetrical with respect to the axis of the long side ofthe waveguide.

What is claimed is:
 1. A microwave system comprising: a microwavegenerator (10); a rectangular waveguide (110) coupled to the generator(10), matched in order to operate in the fundamental mode (H10) or inthe transverse electric mode (TE10), and combined with means ensuringstanding wave conditions; a plurality of power output ports (116) placedin the waveguide (110) in the regions of maximum amplitude of one of thecomponents of the electromagnetic field in order to provide powerdivision for the generator (10), the power output ports (116) beingadjusted in such a way that the sum of their reduced admittances broughtback to the input of the divider formed by the rectangular waveguide(110) is unitary a plurality of implements in form of sources (400)operating with a supplied microwave power, which are coupledrespectively to an output port (116) of the waveguide (110) a pluralityof isolator means (200) provided respectively between an output port andan implement for ensuring power transmission from the output port (116)to the implement (400), without being reflected back to the output port(116), and a plurality of devices (300) respectively associated witheach implement for matching the impedance of each implement (400), eachdevice being located downstream of an isolator means (200), between thelatter and an associated implement (400).
 2. The system as claimed inclaim 1, characterized in that at least one of the implements (400) is aplasma source.
 3. The system as claimed in claim 1, characterized inthat the waveguide (110) includes a reflecting plane (130) forming afixed or movable short-circuit in order to fulfill a standing wavecondition.
 4. The system as claimed in claim 1, characterized in thatthe rectangular waveguide (110) with its output ports (116) is placed ina ring resonator.
 5. The system as claimed in claim 1, characterized inthat the power output ports are formed from electrical antennas locatedat the maximum of the electric field intensity.
 6. The system as claimedin claim 1, characterized in that the power output ports (116) areformed from magnetic antennas located at the electric field intensityminimum.
 7. The system as claimed in claim 1, characterized in that thepower output ports (116) are formed by slots.
 8. The system as claimedin claim 1, characterized in that the power output ports (116) arelocated on a long side (112) of the waveguide (110).
 9. The system asclaimed in claim 1, characterized in that the power output ports (116)are separated by half of the wavelength (λ_(g)/2) in the waveguide(110).
 10. The system as claimed in claim 1, characterized in that itincludes at least one pair of antennas (116) placed respectively oneither side of the axis of the long side (112) of the waveguide (110).11. The system as claimed in claim 1, characterized in that it includesat least one pair of antennas (116) which are placed respectively oneither side of the axis of the long side (112) of the waveguide (110) ina symmetrical manner.
 12. The system as claimed in claim 1,characterized in that it includes at least one pair of antennas (116)which are placed respectively on either side of the axis of a long side(112) of the waveguide (110), respectively on each of the long sides(112) of the waveguide.
 13. The system as claimed in claim 1,characterized in that it includes at least one pair of antennas (116)which are placed in a symmetric manner respectively on either side ofthe axis of a long side (112) of the waveguide (110), respectively oneach of the long sides (112) of the waveguide.
 14. The system as claimedin claim 1, characterized in that the isolator means (200) consists of athree-branch circulator (210), which is combined with a matched load(212) on one of the branches.
 15. The system as claimed in claim 1,characterized in that the impedance matching device (300) is chosen fromthe group comprising a trombone line or a system having three plungers.16. The system as claimed in claim 1, characterized in that, for themicrowave power to be split equally N ways, N antennas (116) ofidentical reduced conductance are provided, each conductance, taken backto the input of the waveguide, being equal to g=1/N.